1. Field of the Invention
The invention relates to a sheet processing apparatus for effecting process on a sheet bundle, particularly a sheet processing apparatus which reduces the occurrence of wrinkles, twists or the like to a sheet bundle when conveyed after process, and an image forming apparatus provided with this sheet processing apparatus in the apparatus main body thereof.
2. Related Background Art
There is, for example, a sheet processing apparatus which binds sheets into the shape of a bundle. Such a sheet processing apparatus is sometimes provided as one of the constituents of an image forming apparatus in the apparatus main body of the image forming apparatus (see Japanese Patent Application Laid-open No. H11-322171). Image forming apparatuses include a copying machine, a printing machine, a laser beam printer and a compound machine of these.
A conventional sheet processing apparatus 901 shown in FIG. 14 of the accompanying drawings is adapted to perform the aligning process of aligning the end portion of a sheet bundle, the side stitching bookbinding of binding the end portion of the sheet bundle, the saddle stitching bookbinding of binding the middle and the vicinity of the middle of the sheet bundle, and then folding the sheet bundle in two to thereby form it into the shape of a brochure, etc.
That is, the conventional sheet processing apparatus 901 receives sheets P having images formed on one side or two sides thereof in the apparatus main body 903 of an image forming apparatus 902 in succession by an intermediate tray 906 and at the same time, aligns the widths of the sheets by a width aligning device 907 to thereby form the sheets into the shape of a bundle. Thereafter, the sheet aligning apparatus 901 staples the edge portion of the sheet bundle by an end portion stapler unit 908, and discharges the sheet bundle to a sheet stacking portion 904 by a pair of sheet discharging rollers 909. Thus, the conventional sheet processing apparatus 901 shown in FIG. 14 can bind the edge portion of the sheet bundle.
Also, the conventional sheet processing apparatus 901 stacks the sheets successively received from the apparatus main body 903 of the image forming apparatus 902 onto a saddle stitching process tray 910 formed substantially straight at a steep slope, and receives them by a stopper 911 and forms them into the shape of a bundle. The sheet processing apparatus 901 effects the width alignment of the sheets P by a width aligning device (not shown) and thereafter staples substantially the vicinity of the middle of the sheet bundle at two locations by an intermediate portion stapler unit 912. Thereafter, the sheet processing apparatus 901 moves the stopper 911 to thereby oppose the stapled portion of the stapled sheet bundle to the nip between a pair of sheet folding rollers 914 and a sheet thrusting plate 913. Then, the sheet processing apparatus 901 thrusts the stapled portion of the sheet bundle by the sheet thrusting plate 913 to thereby feed the sheet bundle into the nip between the pair of sheet folding rollers 914, and folds the sheet bundle into two while nipping and conveying the sheet bundle by the pair of sheet folding rollers 914. Lastly, the sheet processing apparatus 901 discharges the sheet bundle to a sheet stacking portion 905 by a pair of sheet discharging rollers 915. Thus, the conventional sheet processing apparatus 901 shown in FIG. 14 can also form the sheet bundle into the shape of a twofold brochure.
The conventional sheet processing apparatus, however, also nips needles or staples when it discharges the sheet bundle while nipping and rotating the sheet bundle by the pair of sheet discharging rollers 909 in a case where the sheet bundle is bound by the needles and therefore, the pair of sheet discharging rollers 909 have sometimes been damaged.
On the other hand, in recent years, business machines are in the tendency toward a reduction in overlooking area and multiple functions in order to meet a requirement for space saving and users' various requirements. The sheet processing apparatus used also as a business machine are likewise in the tendency toward a reduction in overlooking area and multiple functions. Therefore, even if an attempt is made to contrive a reduction in overlooking area and multiple functions by changing the disposed locations of the end portion stapler unit 908, the intermediate portion stapler unit 912, etc., it is often the case that the stapled sheet bundle is conveyed by a pair of rollers as a plurality of conveying means. Therefore, when the sheet bundle is being conveyed by the pair of rollers, it has sometimes happened that the air collected between adjacent sheets of the stapled sheet bundle is gathered into the stapled portion to thereby form a slack in the sheets. When the sheet bundle is conveyed with a slack thus formed in the sheets, wrinkles or twists have sometimes occurred to the sheets.